Switching relay with 2N7000 and Raspberry Pi GPIO

Question

I'm debugging a circuit on a home project and hoping for some advice.

I have a relay set up like so:

The behavior I'm seeing:

• When I toggle the Raspberry Pi GPIO to HIGH, The relay will begin to thrash (turn on and off very quickly).
• If I take the GPIO line and connect it directly to a 3.3V source from the RPi, the relay turns on cleanly. Disconnecting it from the source turns it off cleanly.
• Changing the RPi GPIO pin, (or changing the entire RPi module) still exhibits the thrashing, so I don't believe its a bad GPIO pin.
• I swapped the relay out for a g5le-1-asi dc3 (It is still set up to be driven by a 5V supply), thinking I was on the operating threshold. However the relay will still thrash when the 2N7000 is driven by a GPIO out.

From the datasheet, I understand the 2N7000 should have no problem being driven by the 3.3V RPi output. But Plugging it directly to a 3.3V source vs the 3.3V GPIO seems to suggest its an issue with the RPi's ability to deliver enough current through the GPIO output. Could this be correct?

What would be the recomended way to drive this relay?

Edit This is the full schematic. RTN is connected to the RPi GND and the RTN on the flyback transformer.

• The RPi is powerd off of the J3 connection
• I have also powered the RPi off of an independent power supply
• The GPIO and RPi ground pins are connected to J4

Edit 2

Everything is laid out on a PCB board with this design:

Answer 1

Your first relay's coil current is 80 mA, which is the problem. The second relay's coil current is 133 mA, which is worse.

The GPIO pin output current capability is not the problem. It's the output voltage. Assuming the output is not a PWM signal . . .

A 2N7000 threshold voltage can be as high a 3 V. At this voltage the FET is rated to conduct only 1 mA. The uC GPIO pin's output voltage is less than 3.3 V, which is so low that it is not fully enhancing ("turning on") the FET. Thus, the FET is a significant resistance in series with the relay coil, not the minimum Rdson shown on the datasheet. The small voltage difference between the GPIO high output and a direct 3.3 V connection is enough to lower the FETs resistance just enough to stop the relay armature from chattering.

IOW, the FET is being starved.

The second relay has a lower operating voltage, but a higher operating current, so it still chatters.

The solution is to change to a bipolar NPN transistor as a saturated switch. Use a base resistor of less than 470 ohms. Go back to the original relay with the lower coil current.

UPDATE:

You want to drive the BJT into saturation. The rule of thumb for this is that the base current should be at least 10% of the desired collector current. For an 80 mA relay, that is 8 mA of base current. The voltage across the resistor is the GPIO pin high voltage under load, around 3 V, minus the Vbe of the transistor. Use Ohm's Law to calculate the resistance, and use the next highest standard value. Of course this works only if the pin can source 8 mA; not all output pins can.

That "rule" is from the 1950's, when transistors were ... worse. By the late 60's and the 70's, things were significantly better, especially for small signal transistors such as the 2N2222. If you don't want to suck that much current out of the GPIO pin, use a ratio of 15:1 or even 20:1. Even if this adds a few tenths of a volt to the transistor's saturation ("on") voltage, the relay still will have 4.5 V across a 5 V coil, and will snap on crisply.